The present invention will be described with relation to its use in the building and construction industry for producing outdoor weatherable plastic siding panels made from vinyl (polyvinylchloride or PVC). The following description relating to vinyl siding panels is an example of one use of the invention for producing outdoor weatherable embossed decorative surfaces on extruded plastic sheets.
Wood, metal and vinyl are materials commonly used as boards or shingles for siding in the construction and remodeling of commercial and residential structures. Painted wood is perhaps the most aesthetically pleasing of these materials to the consumer, but wood suffers from deterioration by rotting and attack by insects, rodents and birds. Wood surfaces need constant reprotection with paint, stains and water sealants.
Advantages of metal over wood include the ability to provide a baked-on finish which is more resistant to the elements than coatings applied to wood, thus eliminating subsequent material and labor costs to the consumer. However, metal-based siding has less thermal insulation value and is sensitive to scratching and denting by hail and other blows to the surface. In addition, it does not have the pleasing look of a wood surface.
Vinyl siding has an economic advantage over most high quality wood and metal siding. It has better insulating qualities than metal, and it is more resistant to denting and scratching. However, vinyl is sensitive to degradation from ultraviolet rays of the sun. Previously, vinyl siding has suffered aesthetically compared to wood because of the generally higher gloss or uniform sheen of vinyl siding, which has a rather unnatural or plastic look. In addition, it is difficult to decorate vinyl surfaces with a natural-looking wood grain pattern that is pleasing to the consumer.
The present invention overcomes the drawbacks of presently available vinyl siding by providing a process that economically produces a vinyl-based siding panel with a superior outdoor weatherable surface having the look of either a painted wood surface or natural unpainted wood grain.
A vinyl siding presently on the market is made by a process in which an extruded sheet of PVC is decorated and embossed to produce a wood grain appearance. A silicone-coated release paper is used in this process to transfer a printed wood grain pattern to the vinyl. Typically, an acrylic clear coat and a color coat with a wood grain pattern are coated on the paper carrier sheet and then transfer-laminated onto the vinyl extrusion under heat and pressure. The wood grain color coat can simulate the appearance of painted or unpainted wood. The clear coat and wood grain color coat bond to the extrusion and release from the paper carrier sheet so that the clear coat forms a protective outer coating for the underlying wood grain color coat. During the transfer process, the release paper is pressed into contact with the extrusion by an embossing roll which transfers deep embossments to the surface of the transferred decorative wood grain. The deep embossments or indentations produce a "shadow" effect in the finished surface that more truly resembles the sheen of natural wood. Without these embossments, the vinyl wood grain-simulating surface has a flat appearance at different light angles--an unnatural look that gives the unsatisfactory appearance of a plastic simulated wood panel.
There are several drawbacks to this prior art process of making vinyl wood grain panels. A principal disadvantage is that gloss levels produced by this process are too high. Surface gloss can be measured by various techniques; and according to one technique (ASTM designation D 3679-86, 5.11), surface gloss levels produced by the prior art process generally have a 75.degree. gloss reading from about 40 to about 50 gloss units. A much lower matte surface with a gloss reading below about 20 gloss units, and preferably below about 12 gloss units, is desirable to produce a more natural looking simulated wood grain finish.
In addition to its surface gloss problem, the silicone-coated paper carrier sheet of the prior art can prevent carefully controlled printing of the decorative coatings. The carrier sheet also should release freely from the decorative layer transferred to the extruded vinyl sheet. Transfer of the decorative wood grain layer to the vinyl sheet takes place at high temperatures, since the decorative coating may be transferred to the heated vinyl extrusion immediately after it exits the extruder die opening. The vinyl sheet is also embossed when the wood grain coat is transferred, and embossing temperatures should be high in order to physically form the embossments. A substantial reduction in temperature of the vinyl extrusion prior to embossing can inhibit forming the deep embossments. If the transferred wood grain coat has a highly roughened surface, in order to produce lower gloss, the surface roughness can interfere with proper release of the carrier sheet. At the high transfer and embossing temperatures, certain coatings on the carrier sheet can preferentially adhere to the carrier rather than properly releasing from it.
The decorative coatings transferred to a vinyl sheet to provide a wood grain or other surface finish also should protect the underlying vinyl panel from UV degradation, delamination, and other weather problems.
Thus, there is a need for a process for manufacturing decorative wood grain vinyl sheets with a sufficiently low gloss surface to resemble the appearance of natural wood grain, painted or unpainted. In a transfer process in which the wood grain coating is printed on a carrier and transferred from the carrier to a vinyl extrusion, while the vinyl is being deeply embossed, there is a need to ensure that the decorative wood grain coat properly releases from the carrier and bonds to the vinyl. The deep embossing step must not be adversely affected; the low matte surface of the decorative wood grain must not adversely affect proper release from the carrier; and these steps must be carried out at the high temperatures and pressures necessary for proper embossing.
One aspect of this invention provides a process for making decorative wood grain extruded vinyl sheeting suitable for outdoor use, in which the surface of the decorative wood grain has a sufficiently low gloss to resemble natural wood grain. The decorative wood grain surface can be deeply embossed to produce a natural looking surface. The low gloss wood grain coat transfers to an extruded vinyl sheet and properly bonds to the vinyl extrusion, while the carrier on which it is coated properly releases from the transferred decorative surface at elevated temperatures. The invention also provides extruded plastic siding panels with weatherable decorative surface coatings that inhibit long term UV deterioration and delamination problems.